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Green Invasion

The trouble with invasive plants


Invasive plants are lurking in our communities and impacting our environment, health, and economy. Here's a list of the worst plants and how to get rid of them.

Invasive species are the second largest threat to biodiversity following habitat loss. Many invasive plants are lurking in our communities and negatively impact our environment, health, and economy. Knowing how to recognize and prevent the spread of these plants is an important step in preserving nature’s delicate balance.

Plants can become invasive when they are introduced into an environment in which they are not indigenous. North America has a long history of species introductions because of the early settlers who introduced many plants as ornamentals or for food and animal forage. Others arrived accidentally in livestock feed, bedding, packing material, clothing, and ship ballast.

Nature’s bullies

Today, globalization and trade allow for the arrival of new species, and in the absence of their natural predators, these plants can grow out of control and outcompete native species, threatening the loss of entire ecosystems. Some invasive plants pose significant health risks: giant hogweed’s toxic sap can cause severe burns, blisters, and scarring in the presence of sunlight, while leafy spurge is toxic to cattle that may accidentally graze on it.

Choking out the economy

The economic effects are significant—eradication and management of purple loosestrife alone costs Alberta, Saskatchewan, and Ontario over $200,000 annually. In Canada, losses associated with invasive species are estimated to be in the billions of dollars every year.

Operation eradication

Many invasions can be stopped or managed early on, and there are several nonchemical methods for doing so. Frequent mowing is very effective, as it keeps the plant from photosynthesizing, ultimately starving the roots. Hand pulling is also effective, but plants such as Japanese knotweed can re-establish themselves from any remaining root fragments.

Some invasives can be “cooked out” by covering the infested area with thick UV-stabilized tarps for a minimum of two years. This method blocks sunlight for photosynthesis and raises the temperature of the ground so that the roots of the plant are killed.

Natural, mechanical methods of management and eradication require persistence and patience. You should never begin mechanical removal of any invasive plant unless you are sure you can commit to the process: it can take more than one growing season to be successful. In the case of plants such as garlic mustard, hand pulling only once can actually increase the problem.

Seek and destroy

Once removed, invasive plants should never be composted, as any living plant material can sprout and grow in compost and create a new population explosion. Instead, bag soft plant material in heavy-duty garbage bags, and let the bags sit in the sun for several weeks before sending them to the landfill. If possible, burn woody plant material.

You can help prevent the establishment and spread of invasive plants by doing the following:

  • Choose native plants over exotic when gardening.
  • Do not use wildflower seed mixes. Many contain non-native, invasive plant seeds, and the packages rarely list the names of all the species contained in the mix.
  • Never dump aquarium materials into local bodies of water. Many invasive aquatic plants are introduced this way.
  • Shop at local nurseries and ask about the origin of the plant you are interested in purchasing. If your question cannot be answered, do not purchase the plant.
  • If your local nursery is selling a known invasive plant, voice your concern.
  • Mobilize your community to get involved. When tackling invasive plants, many hands make light work.

Want to know more? Check out these online resources to find out more about invasive plant species in your region.




Invasive plants to watch out for
(contact your regional invasive species council if sighted)


Identifying features


Special precautions

Canada thistle (Cirsium arvense)

Grows 0.5 to 1.5 m tall; small, urn-shaped purple, pink, or white flowers; slightly hairy stem; lance-shaped, shiny, dark green leaves.

Repeat mowing and hand pulling over several years.

garlic mustard (Alliaria petiolata)

Biennial, with heart-shaped or round leaves with scalloped edges; in second year stalks reach nearly 1 m tall and bear clusters of small, white, four-petalled flowers; stem and leaves sparsely hairy; new leaves have pungent garlic odour.

Hand pulling, ensuring removal of entire root, is effective, as is cutting the shoots at ground level.

Bag uprooted plants immediately if they have already flowered, as seeds may still develop; send bagged plants to the landfill after several weeks of sun exposure.

giant hogweed (Heracleum mantegazzianum)

Easily confused with cow parsnip. Grows up to 5 m tall; flower heads consist of small white flower clusters, up to 1.5 m wide; stem is hollow, with stiff bristles; leaves are large and shiny, deeply serrated with tropical appearance; blooms mid-August, unlike cow parsnip, which blooms in July.

Cut shoots from the roots in early spring when plants are small and easy to handle. In summer, try to prevent spread by removing developing flowers (spreads by seed only).

Sap is highly toxic, and causes photodermatitis. Ensure eyes and skin are protected when handling any part of the plant.

Japanese knotweed (Polygonum cuspidatum)

Up to 3 m tall; jointed, hollow, bamboo-like stems; fleecy white-green flowers appearing late summer. Dig out entire root system and monitor area to remove new plants that arise from root fragments; cut stems repeatedly during growing season to deplete food reserves; cover infested area with black plastic for several growing seasons.


leafy spurge (Euphorbia esula)

Grows up to 1 m tall; small, yellow-green flowers surrounded by two heart-shaped, green bracts.

Till intensively—at least 10 cm deep—every three weeks from emergence in the spring until the ground freezes; may need to continue into the next growing season.

Blue-green stem produces milky-latex that is a skin irritant; wear gloves when handling.

purple loosestrife (Lythrum salicaria)

Up to 1.5 m tall; stems woody and square-ish; deep pink-purple flowers.

Hand pull young plants; cut shoots of older plants; bag or burn onsite.

Early detection and elimination of new infestations is the most effective means of control. Once established, nonchemical methods of removal may be ineffective.


No Proof

No Proof

Raise a glass and say cheers to not-so-hard drinks

Matthew Kadey, MSc, RDMatthew Kadey, MSc, RD